a. Field of the Invention
The instant invention relates generally to devices and methods for ablating tissue. The diagnosis and treatment of electrophysiological diseases of the heart are described in connection with the devices and methods of the present invention. In particular, the instant invention relates to devices and methods related to cardiac ablation for the treatment of atrial fibrillation, as well as the advantages of devices that employ more accurate temperature monitoring during ablation at selected cardiac sites.
b. Background Art
It is well known that atrial fibrillation results from disorganized electrical activity in the heart muscle, or myocardium. The surgical “Maze” procedure has been developed for treating atrial fibrillation and involves the creation of a series of surgical incisions through the atrial myocardium in a pre-selected pattern to create conductive corridors of viable tissue bounded by non-conductive scar tissue.
In performing the Maze procedure and its variants, whether using ablation or surgical incisions, it is generally considered most efficacious to include a transmural incision or lesion that isolates the pulmonary veins from the surrounding myocardium. The pulmonary veins connect the lungs to the left atrium of the heart. The Maze procedures have been found to offer 57-70% success without antiarrhythmic drugs. However, they are also associated with a 20-60% recurrence rate several possible factors, including incomplete lesions, lesion recovery, non-pulmonary vein foci of the arrythymia, or the need for further tissue modifications.
As an alternative to the surgical incisions used in the Maze procedure, transmural ablations of the heart have also been used. The use of catheters for ablating specific locations within the heart has been disclosed, for example in U.S. Pat. Nos. 4,641,649, 5,263,493, 5,231,995, 5,228,442 and 5,281,217. Such ablations may be performed either from within the chambers of the heart (endocardial ablation) using endovascular devices (e.g., catheters) introduced through arteries or veins, or from outside of the heart (epicardial ablation) using devices introduced into the chest. Various ablation techniques have been used, including cryogenic, radiofrequency (RF), laser and microwave ablation. The ablation devices are used to create transmural lesions—that is, lesions extending throughout a sufficient thickness of the myocardium to block electrical conduction, which effectively forms boundaries around the conductive corridors in the atrial myocardium. Perhaps the most advantageous aspect of the use of transmural ablation rather than surgical incisions is the ability to perform the procedure on the beating heart without the use of cardiopulmonary bypass.
Producing precise transmural lesions during cardiac ablation presents significant obstacles for the physician for several reasons. First, the elongated and flexible vascular ablation devices are difficult to manipulate into the complicated geometries required for forming the lesions. Additionally, maintaining the proper positioning against the wall of a beating heart can be difficult. Also, visualization of cardiac anatomy and vascular devices is often inadequate which makes identifying the precise position of such devices in the heart difficult, which can result in misplaced lesions.
During an ablation procedure, precise temperature regulation of the tissue under ablation is another issue encountered by the physician. Tissue ablation generally begins to occur at 50° C., while over-heating occurs at around 100° C. It can be important to monitor the temperature of the tissue during ablation. Most ablation devices accomplish this by measuring the electrode temperature during the ablation. The most common way to monitor the electrode temperature is to install a thermal sensor (e.g., thermocouple or thermistor) inside the tip electrode to measure its temperature.
One of the shortcomings of current methods of monitoring electrode temperature is that the cylindrical shape of a typical ablation catheter tip often means that only a portion of the catheter tip is in direct contact with the ablation surface. Generally, the catheter electrodes themselves do not retain a great deal of thermal energy because they are made of materials with extremely high thermal conductivity, for instance, metals. Thus, the highest temperatures on the electrode itself are generally seen on that portion of the electrode which is direct contact with the tissue surface. Conversely, portions of the electrode not in direct contact with target tissue are most likely contacting blood or interstitial fluids. Since these fluids can conduct a significant amount of heat away from the ablating tissue, temperatures measured in the blood or interstitial fluid may appear to be significantly cooler than the ablating tissue temperature.
Temperature monitoring devices that rely on measuring a point temperature on an electrode with only a single sensor can skew tissue temperature readings by other mechanisms. Since an ablation electrode tip can be quite long in comparison with the diameter of a catheter, the contact angle between the catheter and the ablating tissue can also be a significant factor. For instance, consider the case of a catheter electrode that is positioned with only its distal end contacting on the tissue. If the temperature sensor is located on the proximal end of the electrode (which is not in contact with the tissue) then a significantly different temperature reading from the one present at the tissue may be measured. Whether the result of tissue-contacting angle or conduction of heat away from the electrode, the monitoring of only a single point temperature can produce misleading readings of the temperature at an ablation surface.
If presented with an inaccurate temperature reading, a physician may apply an inappropriate amount of ablation energy to the site. If a tissue being ablated does not reach a sufficiently high temperature for a sufficient period of time, the target tissue may receive an incomplete ablation, which can significantly affect the efficacy of the treatment. On the other hand, excessively high electrode temperatures can cause tissue steam-pop, tissue-charring, or other serious over-heat related tissue damage. Additionally, blood contacting an overheated electrode can lead to the formation of coagulum, which can present thrombo-embolitic hazards to the patient.
The limitations of single point temperature monitoring are well known and several attempts have been made to overcome their disadvantages. U.S. Pat. No. 6,312,425 (Simpson, et. al.) discusses a design using multiple thermal sensors to monitor the temperature in a tip electrode. Simpson describes an arrangement of thermal sensors where one sensor is positioned at the distal apex of the tip and a plurality of other thermal sensors are positioned around the circumference of the electrode in a more proximal location on the catheter. Since the highest temperatures are most likely to be found at the portion of the electrode making contact with target tissue, by monitoring and comparing the temperature reading at the distal end of the electrode to that at the proximal end, a determination can be made of both the maximum temperature being experienced by the tissue and which end of the electrode is performing the ablation.
Similarly, U.S. Pat. No. 6,616,657 (by the same inventors) discusses a method of using the multiple temperature sensors to determine the orientation of the catheter. Experience with that device has shown that the variation in temperature between the end in contact with tissue and the non-contacting end is approximately 10° C. Therefore, an ablation which is showing a temperature at the distal apex of the electrode that is 10° warmer than the proximal sensor is assumed to be in an “end-firing” position—with the distal end of the catheter providing all of the ablation energy to the tissue. Similarly, an observation that the proximal ports are 10° warmer than the distal sensor means that the electrode is in a “side-firing” position—with only the side portion of the catheter contacting the tissue. Temperature differences falling somewhere between 1-10° are assumed to indicate a contacting angle between these two extremes.
The thermal sensors described in each of the above publications are positioned at different ends of a relatively long electrode. However, it is often desirable to use a short electrode during an ablation procedure in order to improve RF energy transfer efficiency. A typical short electrode of this type has an electrode length of approximately 2.0-2.5 mm. This length is roughly comparable to the diameter of most ablation catheters. As mentioned previously, most ablation electrodes are made from materials having excellent thermal conductivity. In these situations, placement of temperature sensors at the proximal and distal ends of a relatively short electrode may create an undesirable situation where the temperature differences between the sensors will not be significant enough to measure because of the rapid conduction of heat energy from one end of the electrode to the other.
What are needed, therefore, are devices and methods which allow for precise temperature measurement at several points in relatively short ablation electrodes. The devices and methods would preferably allow the physician to accurately measure the maximum temperature experienced by a target tissue during a thermal ablation and provide information that would allow the physician to determine the extent and orientation of contact between the ablating electrode and the target tissue.